Docs: Add Material.format. (#22459)

docs/api/en/materials/Material.html

@@ -128,6 +128,11 @@
 		When drawing 2D overlays it can be useful to disable the depth writing in order to layer several things together without creating z-index artifacts.
 		</p>
 
+		<h3>[property:Number format]</h3>
+		<p>
+		When this property is set to [page:Textures THREE.RGBFormat], the material is considered to be opaque and alpha values are ignored. Default is [page:Textures THREE.RGBAFormat].
+		</p>
+
 		<h3>[property:Boolean stencilWrite]</h3>
 		<p>
 		Whether stencil operations are performed against the stencil buffer. In order to perform writes or comparisons against the stencil buffer this value must be *true*. Default is *false*.

docs/api/zh/materials/Material.html

@@ -107,6 +107,10 @@ Default is *false*.
 	在绘制2D叠加时，将多个事物分层在一起而不创建z-index时，禁用深度写入会很有用。
 </p>
 
+<h3>[property:Number format]</h3>
+<p>
+When this property is set to [page:Textures THREE.RGBFormat], the material is considered to be opaque and alpha values are ignored. Default is [page:Textures THREE.RGBAFormat].
+</p>
 
 <h3>[property:Boolean stencilWrite]</h3>
 <p>